It is th long range purpose of this work to clarify the nature of cellular molecules responsible for the malignant phenotype of chemically transformed cells. Although many attempts have failed to identify such a molecule, recent progress in molecular biology and somatic cell genetics have provided new methods to isolate and characterize the cellular genes and their products which seem very useful for this objective. Two approaches are taken. One approach is an attempt to isolate cellular genes which directly control the expression of transformed phenotype and another approach is to clarify the structure and expression mechanism of the genes coding macromolecules, alterations of which have been suggested to be associated with the expressson of transformed phenotype. Preliminary results indicate the possibility to isolate genes controlling the transformed phenotype of human cells by using transfection and gene cloning techniques. Actin, a major protein in the cytoskeleton, has been considered to be involved in many cellular functions including expression of transformed phenotype. An altered form of actin molecule was found in the chemically transformed human fibroblast cells. cDNA synthesized from actin mRNA of these human cells was inserted into plasmid pBR322 DNA.